Abstract
The reactivity of a rotaxane that acts as an aminocatalyst for the functionalization of carbonyl compounds through HOMO and LUMO activation pathways has been studied. Its catalytic activity is explored for C-C and C-S bond forming reactions through iminium catalysis, in nucleophilic substitutions and additions through enamine intermediates, in Diels-Alder reactions via trienamine catalysis, and in a tandem iminium-ion/enamine reaction. The catalyst can be switched "on" or "off", effectively controlling the rate of all of these chemical transformations, by the in situ change of the position of the macrocycle between two different binding sites on the rotaxane thread.
Highlights
Enzymes are often regulated in nature through trigger-induced effects,[1] processes that are inspiring a growing number of synthetic catalysts for which an external stimulus can turn “on” or “off” the catalytic activity[2] or change the stereochemical outcome[3] of the system
Understanding how switchable catalysts behave in their different states and how they participate in different reaction types could be important for their applications: for example with pools of differently functionalized building blocks that can react with the catalyst through different pathways or with catalysts where the “off” state leads to a reduced but not a zero level of catalytic activity with reactive substrates
We recently described a switchable organocatalyst based on a rotaxane (1) able to control the rate of the Michael addition of an aliphatic thiol to trans-cinnamaldehyde (Figure 1).[4]
Summary
Enzymes are often regulated in nature through trigger-induced effects,[1] processes that are inspiring a growing number of synthetic catalysts for which an external stimulus can turn “on” or “off” the catalytic activity[2] or change the stereochemical outcome[3] of the system. When the rotaxane is protonated the macrocycle preferentially interacts with the ammonium unit (a better binding site than the triazolium ring), concealing it from the bulk, and the catalyst is in its “off” state (Figure 1, top).[7] When the secondary amine is not protonated the triazolium ring is the preferred binding site for the macrocycle so the catalytic center is exposed and available to participate in catalysis (Figure 1, bottom).[8]. It was of interest to investigate both the ability of acyclic secondary amines to catalyze a variety of reaction types through different activation modes and the effectiveness of switching “off” catalysis through encapsulation using a rotaxane architecture. Through the acid/base-promoted change of the position of the macrocycle between the different binding sites on the axle, revealing or concealing the organocatalytic dibenzylamine/ammonium group
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